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Pi electrons in conjugated molecules: How mysterious are they?
In theoretical chemistry, a "conjugated system" refers to a system with delocalized electrons in connected p orbitals. This combination generally reduces the overall energy of the molecule and increases stability. The classic manifestation of a conjugated system is alternating single and double bonds. According to the term coined by German chemist Johannes Thiele in 1899, conjugation occurs when p orbitals connected by adjacent σ bonds overlap. Typically, one applies this property to molecules because the combined π electrons do not belong to a single bond or atom, but to a group of atoms.
Conjugated molecules allow electrons to flow more freely due to the overlap of adjacent p orbitals, thereby forming a more stable co-borrowing system.
In a conjugated system, in addition to the traditional p orbital combination, other components such as isolated electron pairs, free radicals or carboxyl cations are involved. These conjugated molecules can be cyclic, acyclic, linear or mixed. Common organic conjugated molecules include 1,3-butadiene, benzene, and allyl cations, while the largest conjugated systems are found in graphene, graphite, conductive polymers, and carbon nanotubes.
Chemical bonding in conjugated systems
Conjugation is usually achieved by alternating single and double bonds, and each atom provides a p orbital perpendicular to the plane of the molecule. Even complex molecules like furan have two alternating double bonds in this five-membered ring, one on each side of the oxygen. One of the oxygen's lone pairs maintains its overlap in the p orbital at that position, thus maintaining the conjugated connection. However, not all lone pairs will participate in conjugation. For example, in pyridine, the nitrogen atom is already included in the conjugated system through a double bond with an adjacent carbon, so the lone pair is on a plane and does not participate in conjugation.
Conjugated systems must be planar (or nearly planar), so the participating lone pairs occupy orbitals with pure p properties rather than the spn hybrid orbitals typical of non-conjugated lone pairs.
Stabilizing energy
Quantitative estimation of the stabilization energy of conjugation is quite controversial because it depends on the assumptions underlying the comparison to the baseline system or reaction process. When the energy of conjugation is formally defined, we call it the resonance energy, which is the energy difference between the actual chemical and a hypothetical dedicated π bond. Although this energy cannot be measured directly, there is generally some consensus that cationic systems are generally much more stable than neutral systems.
Interestingly, when it comes to polyconjugates, such as benzene, the resonance energies of these species range from around 36–73 kcal/mol, showing the large contribution that conjugation makes to chemical stability.
Blue pigment and color
In compounds with conjugated π systems, electrons are able to capture specific photons, similar to a radio antenna detecting photons along its length. In general, the more conjugated (i.e. the longer the π system), the longer the wavelength of photons it can capture. Molecules that absorb light in the visible range often exhibit color, especially when they contain more conjugated bonds. Common colors include yellow or red.
For example, in beta-carotene, the long conjugated hydrocarbon chain is responsible for its intense orange color, which is due to its electronic excitation, which is promoted to a higher energy state when the system absorbs photons of specific wavelengths.
The connection between stability and structure
The stability of conjugated molecules often reveals subtle relationships between structure and reactivity. Through the delocalization of electrons and the quantum mechanical properties of various species, researchers can unveil the veil of these mysterious molecules. As our understanding of conjugated systems deepens, we can't help but ask, what kind of secrets are hidden in these seemingly ordinary chemical structures?
